1. Field of the Invention
The present invention relates to a semiconductor memory device which includes a circuit including a semiconductor element such as a transistor.
2. Description of the Related Art
Semiconductor memory devices are roughly classified into two categories: a volatile device that loses stored data when power supply stops, and a non-volatile device that retains stored data even when power is not supplied.
A typical example of a volatile semiconductor memory device is a dynamic random access memory (DRAM). In the DRAM, a transistor is selected and electric charge is accumulated in a capacitor connected to the transistor, whereby data is stored.
In the DRAM, electric charge accumulated in the capacitor is lost as data is read out, and thus rewriting of data is needed every reading of data. Further, the electrical charge is also lost due to, for example, a leakage current (off-state current) between a source and a drain of the transistor in the DRAM when the transistor is off; therefore, the data retention period is short. Accordingly, a writing operation (refresh operation) needs to be performed at certain intervals, which increases power consumption. Further, since data is lost when power supply stops, another type of memory device using a magnetic material or an optical material is further needed to retain data for a long period of time after power supply stops.
Another example of the volatile semiconductor memory device is an SRAM (static random access memory). The SRAM retains stored data by using a circuit such as a flip-flop and thus does not need a refresh operation, which is an advantage over the DRAM. However, cost per storage capacity is higher because the circuit such as a flip-flop is used. Further, as in the DRAM, stored data in the SRAM is lost when power supply stops.
A typical example of a non-volatile semiconductor memory device is flash memory. The flash memory includes a floating gate between a gate electrode and a channel region of a transistor and stores data by holding electric charge in the floating gate. Therefore, the flash memory has advantages in that the data retention period is extremely long and a refresh operation, which is needed in the DRAM, is not needed (e.g., see Patent Document 1).
However, a gate insulating film included in the flash memory is deteriorated by tunneling current generated in data writing, and thus a certain number of data writings cause a failure of the semiconductor memory device. To suppress an adverse effect of this problem, for example, a method of equalizing the number of writing operations between memory cells is employed, in which case a complicated peripheral circuit is needed. Further, such a method does not solve the fundamental problem of lifetime. Therefore, the flash memory is not suitable for applications involving frequent data rewritings.
Further, the flash memory needs high voltage for injecting electric charge in the floating gate or removing the electric charge. A circuit for that purpose is also needed. In addition, it takes a relatively long time to inject or remove the electric charge, and thus it is not easy to increase the speed of writing and erasing data.
Transistors included in the above-described semiconductor memory devices use mainly a silicon semiconductor. A transistor using an oxide semiconductor, which has drawn attention in recent years, exhibits higher field-effect mobility than a transistor using amorphous silicon. Therefore, such a transistor using an oxide semiconductor has been expected to replace the transistor using amorphous silicon.